Perimeter insulation - flooring butted up to or laid over?

[Dunc]

Hopefully a quick one: When installing the PIR floor insulation a 50mm perimeter upstand is specified. In one of the architects diagrams the upstand stops level with the screed and the final floor covering oversails this to butt up aginst the wall. In another diagram the perimeter upstand extends high enough for the floor covering to be separated from the outisde wall.

 

As this is a larsen truss frame, the inner sole plate is effectively insulated from the outside as it doesn't span the full width of the wall.

The blockwork underneath the sole plate has a course of Marmox blocks below the top block and this will overlap with the 220mm PIR, so the top block is thermally broken from the ground....so I *think* it's OK to oversail the upstand?

 

Top to bottom:

Tiles

50mm screed with UFH. Top of screed is level with the bottom of the sole plate

slip membrane

220mm PIR

150mm reinforced slab.

 

 

 

 

 

 

 

 

 

Edited December 5 by Dunc
adding diagrams

[Oz07]

You may as well go with bottom drawing. No need for floor finish to go any further than the line of your finished wall and looks like your wall is in from the timber frame?

[JohnMo]

Took my insulation at the wall edge out as far as the inside edge of the skirting. So bottom image but wider.

 

I would do 200mm PIR and increase screed depth.

[Gus Potter]

1 hour ago, Oz07 said:

You may as well go with bottom drawing. No need for floor finish to go any further than the line of your finished wall and looks like your wall is in from the timber frame?

Plus 1 to that. Once you take into account the build sequence then agree second option is more practicable. 

 

I know you have just asked about the inside but can you just check the weathering detail on the outside?

 

This is a critical detail for longevity. Ask your Architect if they know whether the Larsen truss has been designed so both the inside and outside flanges require structural support.

 

@Dunc Does your cladding require ventilation behind, always worth checking the fire protection detail around doors , windows and at the wall head if this is the case.

[Dunc]

@Gus Potter thanks for looking at the other details.

The load transfer is on the inner leaf, aparantly.

Vertical timber cladding has 20mm vertical battens then 42mm horizontal so there should be plenty ventilation.

 

[Bonner]

13 hours ago, JohnMo said:

I would do 200mm PIR and increase screed depth

Agree, 50mm screed is absolute minimum and leaves no margin for variations

[torre]

2 hours ago, Bonner said:

50mm screed is absolute minimum

If this is a liquid screed then 50mm is fine, some are even 40mm

[JohnMo]

40 minutes ago, torre said:

If this is a liquid screed then 50mm is fine, some are even 40mm

The reason I said in the first place was, that the flexibility comes from having a big energy store, then you can play tunes with how you heat your house. Pure weather compensation is fine with thin and thicker screeds, batch charge can only be done with thicker screeds.  Batch charging can be useful with time of use tariffs. A few on here only heat on cheap rate periods.

 

40mm is more like a big radiator.

[Dunc]

I'm well aware of the arguments around thin and thick screeds. There are many excellent discussions on this forum covering the subject.

We're going at 50mm (architect was pushing for 40mm!). The PIR is already on site and the first layer (120mm) installed so rightly or wrongly, we're fully committed.

I would, in retrospect, have preferred 70mm to allow better tolerance of the uneven slab but we are where we are and I'm sure it'll work out.

Everyones thoughts and recommendations have been much appreciated as always!

[saveasteading]

5 minutes ago, Dunc said:

70mm to allow better tolerance of the uneven slab

How uneven? 40mm screed should be the minimum and some slab layers are hopeless at levels..

[Dunc]

3 hours ago, saveasteading said:

How uneven? 40mm screed should be the minimum and some slab layers are hopeless at levels..

Will have to wait until its all in and get the laser level on it, but looks like +/- 5mm at the moment, so not disastrous!

[Gus Potter]

On 06/12/2025 at 08:07, Dunc said:

The load transfer is on the inner leaf, apparantly.

There is a discprecancy here. Go back and check what parmeters the Larsen truss designer was given. Ask; how do the Larsen trusses cope with the wind load. Larsen truss design is a bit of an art, it's a lot of fun mind and massively elegant, the wind loads get applied in a different way compared with a standard timber frame. The detail you show indicates that the outer and inner flange of the Larsen truss are restrained. 

 

On 06/12/2025 at 13:09, Dunc said:

We're going at 50mm (architect was pushing for 40mm!). The PIR is already on site and the first layer (120mm) installed so rightly or wrongly, we're fully committed.

It seems that your are on the face of it committed, but if there is an error then no point in compounding this. 

 

If I was you I would be confirming in writing to your Architect that a 40 mm screed is achievable, buildable at reasonable cost and it is going to remain serviceable ( i.e) not cause problems with the flooring. 

 

I've dabbled with this over the years and I feel that you tolerances are too tight and you are in for a nasty shock At least you'll end up paying for something that does not get delivered on site. 

 

My suggestion is that you get together with your Architect.. find out just how much they know about what is deliverable in reality. Give them a way to save face and see if you can change the levels. 

 

Bite the bullet now as later it will likely get worse. 

On 06/12/2025 at 17:12, Dunc said:

Will have to wait until its all in and get the laser level on it, but looks like +/- 5mm at the moment, so not disastrous!

Well it will be a disater if what you are laying on is 5 - 10mm high? so now the pipe cover will be a bit of a joke. As an SE I'm very careful to set buildable tolerance limits. Ask your Architect how they are going to account for the variation in level and flatness of what you are laying on. 

 

In summary you take your chances! As an SE I'm not that fussed if your UFH does not work so well. With my desinger hat on I think this is vital to get it right, even if you have to go back and review the potential desing flaws which I think are there. It does not have to to result in a big falling out!